I've put very little into the gun in terms of upgrades. The sight is a Classic Army, but it's a replica of the US M145 sight rather than the C79. The sight was about $125 or so on ebay including shipping. Externally, the gun is completely stock right now.
Internally, I've installed a Prometheus MS110SP spring, Systema bearing spring guide, Systema polycarbonate piston, and System silent piston & cylinder head. In a couple of days, I'll be ordering a Systema tappet plate and airseal nozzle and either Systema or Prometheus hopup rubber.
Without checking the prices offhand (parts came from airsoftparts.ca or ehobbyasia), I'd say I've probably put about $150 into its internals so far, not counting the final parts I'll be ordering this week.
My final internal upgrade will be replacing the barrel with a Systema tightbore barrel sometime down the road. The only time I plan on replacing any other internals is if something breaks or wears out. Externally, I want to send my lower receiver off for complete sandblasting to remove the laser-etched Armalite trademarks and have it engraved with the Diemaco & Canadian Forces C7A1 markings. And since they're going to have to engrave the selector switch markings again, I want my full auto position to be labelled as "FUCK YEAH".